BUENA PARK, Calif. -- The death of a police officer who collapsed during SWAT training exercises Friday morning has stunned the police department.

Daniel Ackerman, 31, was exhausted after the semiannual physical fitness training in Fullerton and was taken to St. Jude Medical Center, said Lt. Richard Forsyth. Ackerman died at 3:40 a.m. Saturday, officials said.

Ackerman spent more than half his life in and around the 90-officer department and last year got his dream job of becoming a SWAT officer.

"It's devastating to the police department," said Forsyth, a 23-year veteran who met Ackerman as part of the Police Explorer program in October 1995. "It's completely unexpected and very difficult to understand. We're all trying to do the best we can."

Ackerman, who lived in Tustin and was a single father of a 2-year-old daughter, is also survived by his mother and sister.

An autopsy to determine the cause of death could possibly take place Monday.

Buena Park officers are wearing black bands across their badges and will continue to do so until full police services for Ackerman, details of which are still to be announced.

"The next few days and months are going to be pretty difficult," said Forsyth, adding that Ackerman appeared to be in good physical health.

There was "nothing to give us any sign or indication that anything like this would happen," he said. "We're all kind of at a loss to piece it together and to understand what happened."

Ackerman became a reserve police officer in April 2001 and was hired full time in June 2005, the department said.

"This was his life," Forsyth said. "He's pretty much spent his last 16 years, more than half of his life, working somewhere here at the police department. This is what he loved to do. ... He died doing what he loved."

Flowers or donations for the family can be made in Officer Ackerman's memory in the care of the Buena Park Police Association, 6640 Beach Blvd., Buena Park, CA 90621.